查询总是返回一个空行？

Question

This query always returns at least one row even if none is found

(
    SELECT accounting.time, enclosure.enc_id_, enclosure.txt, accounting.amount AS sum, SUM(ROUND(vatcode.percent/(100+vatcode.percent)*accounting.amount)) AS sum_vat
    FROM accounting
    INNER JOIN enclosure ON enclosure.id=accounting.enc_id
    LEFT JOIN vatcode ON vatcode.id=accounting.vatcode_id
    WHERE accounting.account_id='10'
)
UNION (
    SELECT accounting.time, enclosure.enc_id_, enclosure.txt, accounting.amount*-1 AS sum, NULL AS sum_vat
    FROM accounting
    INNER JOIN enclosure ON enclosure.id=accounting.enc_id
    WHERE accounting.accountoff_id='10'
) ORDER BY time

I know that the error occurs in the second select here ... , NULL AS sum_vat .. If I remove it I get an error about not having the same statements in both select? How can this be solved?

return

Array
(
    [time] => 0
    [enc_id_] => 0
    [txt] => 
    [sum] => 0
    [sum_vat] => 
)

Answer 1

If you use an aggregate without a group by, the aggregate will run over the entire table, always returning a single row. For example,

select max(price) from items where group = 'Servers'

returns a single row with the highest price. MySQL is the only database that allows other columns without a group by :

select name, max(price) from items where group = 'Servers'

But confusingly, it would just put a random value in name column; the name here won't be the name of the highest priced server.

In your case, the obvious solution is to add a group by to the first part of the union:

SELECT accounting.time, enclosure.enc_id_, enclosure.txt, accounting.amount sum, 
    SUM(ROUND(vatcode.percent/(100+vatcode.percent)*accounting.amount)) sum_vat
FROM accounting
INNER JOIN enclosure ON enclosure.id=accounting.enc_id
LEFT JOIN vatcode ON vatcode.id=accounting.vatcode_id
WHERE accounting.account_id='10'
GROUP BY accounting.time, enclosure.enc_id_, enclosure.txt, accounting.amount